Fighters of the Islamic State of Iraq in Syria parade in Tel Abyad. Syrian rebels' uneasy co-existence with the hardline Isis has turned to outright hostility. Photograph: Reuters

The most serious clashes yet between the Syrian opposition and a prominent al-Qaida group erupted in the north of the country on Friday as a tribal revolt against the same organisation continued to rage in Iraq's Anbar province.

Opposition groups near Aleppo attacked militants from the Islamic State of Iraq in Syria (Isis) in two areas, al-Atareb and Andana, which are both strongholds of the fundamentalist Sunni organisation.

Battles also erupted in the Salahedin district of Aleppo itself, where both groups had reluctantly co-existed during recent months as Isis had imposed its hardline influence on parts of the city. Several hundred miles east, Isis remains in control of parts of the Iraqi cities of Fallujah and Ramadi, having raided mosques, sacked police stations and freed prisoners in moves reminiscent of the darkest days of Iraq's insurgency, in which much of Anbar had been lost to al-Qaida.

Isis is the latest incarnation of the same ruthless group that held sway in Anbar before the Awakening Movement of tribal militias ousted it. The Awakening was led at the time by powerful local sheikhs and backed by the occupying US military and was credited with freeing both cities from the grip of the jihadists.

But over the past year, security there and elsewhere in Iraq has gradually ebbed as the war in Syria has intensified. In the past week, revitalised Isis insurgents stormed into both cities soon after the Iraqi military withdrew from a violent standoff with local tribes.

The same group has been at the vanguard of an increasing radicalisation of the anti-Assad opposition in northern Syria. Its members cross freely between Anbar and the eastern deserts of Syria as the insurgencies in each country steadily seep in to each other.

Tribal figures in Anbar said they were continuing to mount attacks on Isis and were determined to block the Islamists' efforts to re-establish a foothold there.

"Never will we allow them to return to our towns," said a senior sheikh from the outskirts of Ramadi. "We don't trust the Shia regime of Maliki and we don't trust al-Qaida. We will fight for our futures. No one else has our benefit at heart."

The US military had placed great significance on Ramadi and Fallujah, having fought two major battles against insurgents in Fallujah in 2004 and having suffered more than one third of its casualties during the eight-year war in the restive province.

With the US having left Iraq three years ago, the government of the Iraqi prime minister, Nouri al-Maliki, recently travelled to Washington to seek renewed American intelligence help to get on top of the insurgency. The Obama administration agreed to supply weapons and technicians but it is not yet clear if it also agreed to re-introduce elements of its controversial drone programme.

Though not thought to be co-ordinated, the attacks on Isis strongholds in Syria and Iraq have mounted the most serious challenge to the group's authority since it again became a dominant player in the region.

The group's members have imposed a strict interpretation of Islamic law in much of northern Syria, subverting local authority and intimidating towns and communities. The increasing strength of the group has also further splintered the original armed Syrian opposition, which has at times come to a battlefield accommodation with the better funded jihadis, and had tried to avoid a reckoning with them.

However, opposition leaders told the Guardian that with military momentum at a crawl, they have little option but to try to oust Isis.

"We have surrounded them in Andana," said a leader of Ahrar al-Sham, an Islamic group within the opposition. "We have told their foreigners that they must come and join us, within 24 hours, or face being killed."

In al-Atareb, several dozen fighters, including Isis members, are believed to have been killed in the clashes. The group is thought have at least 10,000 members in northern Syria, many of them foreigners from elsewhere in the Sunni Islamic world, including up to 1,000 Europeans.

Isis has kidnapped more than 30 foreign aid-workers and journalists in the north, along with scores more Syrians. The French medical aid agency Médecins Sans Frontières said five of its members had been taken from a house in northern Syria on Thursday. It gave no details about the identities of the captives, or where they were taken from.